Waste water that contains high concentrations of pollutant substances such as organic substances, phosphorus components, and/or or nitrogen components that increase biological oxygen demand (hereinafter “BOD”) and/or suspended solids concentration (hereinafter “SS”) is a cause of river pollution, red tide, and other forms of environmental pollution. In the past, known methods for treating waste water that contains high concentrations of pollutant substances of this type have been the so-called modified Barnard method which is an aerobic and anaerobic circulation method. With this method, in order that the alkali that is released during denitrification reactions is reused in nitration reactions, the denitrification step in the active sludge treatment step is divided into two stages, a first denitrification tank and a second denitrification tank, which are located before and after the first nitration tank. In addition, in this method, a second nitration tank is provided after the second denitrification tank, and mixed solution flowing out of the first and second nitration tanks is circulated to the first denitrification tank.
However, the following problems arise with waste water treatment carried out using this method.    (1) High-concentration ammoniacal nitrogen has bactericidal properties, and therefore often impedes activation of activated sludge during the activated sludge treatment step. Activated sludge treatment will therefore be insufficient.    (2) With so-called modified Barnard methods, the hydrogen ion concentration (hereinafter “pH”) decreases with progression of the nitration reaction in the nitration tank, but the nitration reaction depends on pH, becoming slower as the pH decreases. As a result, the activated sludge treatment expectedly becomes inadequate, and phosphorus removal and the like will also be insufficient.    (3) When the denitrification reaction slows due to insufficient activated sludge treatment, the residual ammonia and/or nitrate and nitrite ion concentrations will increase. As a result, the BOD that is dependent on these nitrogen compounds will increase, and waste water treatment will become insufficient.    (4) High-concentration activated sludge is required for waste water containing high concentrations of contaminants, and there is thus an unavoidable tendency for increased concentrations of activated sludge suspended solids (hereinafter “MLSS”). For this reason, oxygen supply becomes difficult, and stirring the activated sludge as well as solid-liquid separation at the sedimentation tank become difficult.    (5) When activated sludge treatment is to be used for the denitrification and denitration of wastewater having high nitrogen component levels, problems such as sludge floating due to denitrification and/or pH decrease arise, regardless of whether the organic contaminant concentration is low.
In order to respond to problems with the methods described above, the inventors of the present invention have offered a bioreactor that allows efficient activated sludge treatment of waste water containing high concentrations of contaminants such as nitrogen components, phosphorus components, and organic substances, and a waste water treatment method that uses this bioreactor (patent document 1). This bioreactor has an inner tank with a nitration reaction part and a denitrification reaction part that are linked by turbine blades.